Perpendicular magnetic recording (PMR) has become the mainstream technology for disk drive applications beyond 200 Gbit/in2, replacing longitudinal magnetic recording (LMR) devices. Due to the continuing reduction of transducer size, high moment soft magnetic thin films with a Bs above 22 kG are required for write head applications. Although a PMR head which combines the features of a single pole writer and a soft magnetic underlayer has a great advantage over LMR in providing higher write field, better read back signal, and potentially much higher areal density, PMR still suffers some problems. One of the biggest issues is the head-induced data erasure that is of particular concern since the erasure occurs after writing. This type of erasure is believed to be caused by a remanent magnetization in the main pole layer.
A conventional PMR write head as depicted in FIG. 1 typically has a main pole layer 10 or write pole with a pole tip 10t at an air bearing surface (ABS) 5 and a flux return pole (opposing pole) 8 which is magnetically coupled to the write pole through a trailing shield 7. Magnetic flux in the write pole layer 10 is generated by coils 6 and passes through the pole tip into a magnetic recording media 4 and then back to the write head by entering the flux return pole 8. The write pole concentrates magnetic flux so that the magnetic field at the write pole tip 10t at the ABS is high enough to switch magnetizations in the recording media 4. A trailing shield is added to improve the field gradient in the down-track direction.
Referring to FIG. 2, a top view is shown of a typical main pole layer 10 that has a large, wide portion called a yoke 10m and a narrow rectangular portion 10p called a pole that extends a neck height (NH) distance y from the ABS plane 5-5 to a plane 3-3 parallel to the ABS where the pole intersects the yoke at the neck 12. The main pole layer 10 flares outward at an angle θ from a dashed line 11 that is an extension of one of the long rectangular sides of the pole 10p. PMR technologies require the pole 10p at the ABS to have a beveled shape (as viewed from the ABS) so that the skew related writing errors can be suppressed.
To achieve high areal recording density with PMR technology, key requirements for the PMR writer design are to provide large field magnitude and high field gradient in both down-track and cross-track directions. In practice, these two requirements are often traded off with each other to balance the overall performance. One approach involves optimizing the geometry of the main write pole such as modifying the values for NH and flare angle θ. A short NH or large θ can increase write field magnitude effectively. However, too short of a NH leads to problems of meeting process tolerance during manufacturing while too large of a flare angle θ may cause a large amount of adjacent track erasure because of a large fringe field. In today's commercial PMR writer products, NH is generally greater than 0.1 micron and flare angle θ is kept less than 45 degrees.
A trend in the industry is to increase the recording density and recording frequency which requires a higher saturation magnetic flux density (BS) and higher resistivity (ρ) in the main pole layer than provided by conventional write heads. A low coercivity (HC) is also desirable. A laminated high moment film involving an antiferromagnetic coupling scheme with Ru coupling layers between high moment layers has been described in U.S. Pat. No. 7,057,853 and by Y. Chen et al. in “High moment materials and fabrication processes for shielded perpendicular write head beyond 200 Gb/in2”, IEEE Trans. Magn. Vol. 43, No. 2, p 609 (2007). In the laminated scheme, a high moment material such as a FeCo layer is laminated into several thinner FeCo layers that are separated by non-magnetic layer insertions. When a non-magnetic lamination material such as Ru, Rh, or Cr reaches a certain thickness, a coupling energy is generated such that the magnetization of the FeCo layers on either side of a Ru or non-magnetic layer will align in anti-parallel directions thereby establishing an anti-ferromagnetic (AFC) laminated configuration. Since the magnetization in a FeCo layer is oriented opposite to that of the magnetic moment in the nearest FeCo layer, the remanent magnetization can be reduced. However, the AFC coupling strength of a FeCo/Ru/FeCo configuration is typically large and this type of AFC lamination will inevitably cause a large anisotropy field and low magnetic moment under a low field. Although the coupling strength can be lowered by using a thicker Ru of about 18 Angstrom compared with 7.5 Angstroms, the magnetic moment will be diluted as the non-magnetic content in the FeCo/Ru/FeCo stack is increased.
In non-AFC laminations where the lamination scheme does not involve AFC coupling, the reduction of remanent magnetization must be achieved through demagnetization fields. In this case, the FeCo layer is laminated with some non-magnetic material such as Cu or thick Ru. After patterning, the demagnetization fields will force the neighboring FeCo layers to form a closure-like domain structure to effectively reduce remanent magnetization. However, a thick Ru layer results in an undesirable decrease in magnetic moment for the main pole layer while a thicker Cu layer is typically required in order to effectively break the interlayer coupling between two neighboring FeCo layers sandwiched on either side of the Cu layer. Moreover, Cu is typically easy to corrode which is a disadvantage when considering reliability. Examples of non-AFC laminations are described by Min Mao et al. in “Optimization of high Bs FeCo film for write pole applications”, JAP 97, 10F908 (2005), and by K. Nakamoto et al. in “Single pole/TMR heads for 140 Gb/in2 perpendicular recording”, IEEE Trans. Magn., Vol. 40, p 290 (2004). However, an improved non-AFC laminated film is needed that can function as a seed layer or a main pole layer while satisfying all the requirements of a main pole material including large magnetic moment, small coercivity in both easy axis and hard axis directions, small Hk, and small remanence.
Other related prior art includes the following references. In U.S. Pat. No. 7,214,404, a soft magnetic layer made of a NiFe alloy and at least one of Nb, V, Ta, Zr, Hf, Ti, B, Si, and P is used to reduce the demagnetization energy of an adjacent perpendicular magnetic recording layer in a PMR disk.
U.S. Pat. No. 6,452,763 discloses an inner pinned (AP1) layer with a laminated configuration in which FeCo layers are separated by nano oxide layers such as CoFeO.
U.S. Pat. No. 5,862,021 describes the use of a Co oxide film as a buffer layer below a pinned layer to weaken the magnetic coupling between the pinned layer and a free layer and thereby increase the MR ratio of a magnetoresistive element.
In U.S. Pat. No. 7,173,797, a composite inner pinned layer is employed to increase the MR ratio in a CPP type head and is comprised of a stack represented by FeCo/Cu/ferromagnetic layer/Cu/FeCo where the middle ferromagnetic layer is partially oxidized.
A composite free layer in U.S. Pat. No. 7,057,865 has a CoFe/Ru/CoFe configuration and is formed adjacent to a bias layer made of an antiferromagnetic material. The Ru spacer layer has an appropriate thickness to cause strong anti-parallel coupling in the CoFe free layers.